1. Field of the Invention
The present invention relates to an optical axis polarization type laser interferometer including a reference sphere which forms a reference of measurement, a retro-reflecting means disposed at a measurement object, a laser interference measuring apparatus for outputting a measurement value corresponding to an increase or a decrease in the distance to and from the retro-reflecting means, and a two-axis turning mechanism for turning an emission beam of the corresponding laser interference measuring apparatus centering around the reference sphere, which measures, with the center coordinates of the reference sphere used as the reference, the distance to and from the retro-reflecting means where the optical axes of the emission beam from the laser interference measuring apparatus placed on the two-axis turning mechanism and a return beam become parallel to each other. In particular, the invention relates to an optical axis polarization type laser interferometer capable of remarkably simplifying complexities in mechanism design with respect to the first axis turning mechanism that composes the two-axis turning mechanism.
2. Description of the Related Art
As shown in FIG. 1, an optical axis polarization type laser interferometer (also called a tracking type laser interferometer) has been known, which irradiates a laser beam (called a measurement light beam) 22 from a laser interference measuring apparatus 20 toward a retro-reflecting means (also called a retro-reflector) 12 disposed at a measurement object 10, detects a displacement of the retro-reflecting means 12 by utilizing interference of the laser beam reflected in the return direction by the retro-reflecting means 12 and at the same time executes tracking by using a change in the position of the optical axis of the measurement light beam 22 by the two-axis turning mechanism 30. In the drawing, reference numeral 24 denotes a light source, and 26 denotes a laser beam emitted in the direction completely opposite to a target (12) in order to measure a relative displacement between the surface of the reference sphere 34 installed at the center of the two-axis turning mechanism 30 and the laser interference measuring apparatus 20. Reference numeral 34 denotes a reference sphere which forms a reference of measurement, 36 denotes a carriage for placing the laser interference measuring apparatus 20 on the two-axis turning mechanism 30, and 40 denotes a supporting portion for fixing the reference sphere 34.
Thus, in the optical polarization type laser interferometer accompanying the two-axis turning mechanism 30, it is necessary that a turning mechanism 30 having two axes orthogonal to each other is constructed around the reference sphere 34 that is installed at the center and becomes a measurement reference. However, it is not easy that a simple mechanism of high reliability is achieved with the turning center of the corresponding two-axis turning mechanism 30 aligned with the center of the reference sphere 34.
For example, Japanese Patent No. 2603429 (Patent Document 1) describes, as shown in FIG. 2, a mechanism which is based on a gimbal mechanism. In the drawing, reference numeral 31 denotes the first axis turning mechanism, 32 denotes the second axis turning mechanism, and 50 denotes the base portion of the apparatus.
Also, as shown in FIG. 3, European Patent No. EP0919830A2 (Patent Document 2) describes a two-axis turning mechanism that utilizes the profile of the reference sphere 34 and an optical fiber for guiding light. In the drawing, reference numeral 31R denotes a ring of the first axis turning mechanism 31 that composes the gimbal mechanism of the two-axis turning mechanism 30, 31M denotes a drive motor thereof, 32R denotes a ring of the second axis turning mechanism 32 that composes the gimbal mechanism of the two-axis turning mechanism 30 and 32M denotes a drive motor thereof.
In addition, German Patent Publication No. DE202004007647U1 (Patent Document 3) describes, as shown in FIG. 4, a two-axis turning mechanism 30 of a three-section structure having (1) a reference sphere supporting portion 40 for supporting the reference sphere, (2) a first axis turning mechanism moving portion 31A that composes the first axis turning mechanism 31, and (3) a first axis turning mechanism fixing portion 31B that composes the first axis turning mechanism 31 as well, and all of which being co-axially disposed from the center (1) a reference sphere supporting portion 40, (2) a first axis turning mechanism moving portion 31A, and (3) a first axis turning mechanism fixing portion 31B.
However, since the two-axis turning mechanism described in Patent Document 1 guides light by means of a beam bender as means for guiding light of the laser interference measuring apparatus 20 to the two-axis turning mechanism 30, it is necessary that a considerably complex mechanism design is carried out only for the sake of guiding light from the light source.
Also in Patent Document 2, the mechanism design adopts a configuration in which both two axes surround the reference sphere 34, wherein the design thereof still remains complex. In particular, it is necessary to control the laser interference measuring apparatus via two rings 31R, 32R, which compose the gimbal mechanism, and via a sliding mechanism corresponding thereto in order to change the emission direction of the laser interference measuring apparatus 20. Such a complex mechanism requires a high cost even for only to cause it to be driven as a designed mechanism. Further, in the case of an optical axis polarization type laser interferometer, there may be cases where it becomes necessary for the two-axis turning mechanism 30 to be feedback-controlled in line with movement of a target. In this case, it is necessary to minimize non-linear phenomena such as friction, backlash, etc., which pose a risk of adversely influencing the control, in the stage of mechanism design, wherein the cost is further increased. Therefore, as far as the art according to Patent Document 2 is concerned, this necessitates that a remarkably high cost design is executed in terms of industrial aspect.
On the other hand, the mechanism described in Patent Document 3 is further remarkably simplified in comparison with the mechanism designed in Patent Document 2, wherein the first axis and the second axis orthogonal thereto are made clearly separate from each other in the stage of mechanism design. However, since the reference sphere supporting portion 40 for supporting the reference sphere 34 is separated from each component of the first axis turning mechanism 31, it is necessary to prepare a long reference sphere supporting portion 40 that passes through the first axis turning mechanism 31 at its center. Since the center coordinate of the reference sphere 34 is an important factor that becomes the measurement reference of an optical axis polarization type laser interferometer, the mechanical stability of the reference sphere supporting portion 40 for supporting the center coordinate is an important specification item in terms of mechanism design. For example, it is necessary that unexpected displacement of the center coordinates of the reference sphere resulting from natural frequencies and thermal expansion is managed within an appropriate range. From this perspective, it cannot be necessarily said that the mechanism design described in Patent Document 3 is optimized.
That is, it is not realistic that the installation environment of the optical axis polarization type laser interferometer is remarkably strictly restricted. Therefore, if it temporarily attempts to reduce unexpected displacement resulting from thermal expansion of the reference sphere supporting portion 40, adoption of a material whose coefficient of thermal expansion is low is taken into consideration. Generally, the mechanical feature of a material having a low coefficient of thermal expansion, in particular, the specific rigidity influencing the resonance frequency is low in comparison with mechanical structure materials. It is required that the reference sphere supporting portion 40 according to Patent Document 3 is shaped to be slender, the length-to-width ratio of which is large, wherein it is difficult that the mechanism design is optimized from this perspective.
Further, since the weight of the second axis, an interferometer, and a displacement gauge, which are supported by the first axis turning mechanism 31, is increased, it is necessary to achieve an economical design while satisfying required functions such as turning accuracy, weight, etc.